Pigmented paper coating formulations generally comprise an aqueous synthetic polymer binder dispersion and pigment and may contain other additives typically used in the paper coating art. Illustrative of the polymer binders in the dispersions are vinyl acetate copolymers and interpolymers, including vinyl acetate/ethylene (VAE) and vinyl acetate/alkyl acrylate copolymers and interpolymers, and styrene/butadiene styrene/acrylate copolymers. Such copolymers and interpolymers can also contain other co-monomers such as, for example, a copolymerized ethylenically unsaturated mono- or dicarboxylic acid or other unsaturated co-monomers which can function as cross-linking agents.
U.S. Pat. No. 4,395,499, for example, discloses high strength pigment binders for paper coatings having increased water retention and stability. The coating compositions contain an aqueous synthetic polymer latex comprising a dispersed interpolymer of a vinyl ester; a polyethylenically unsaturated co-monomer which can be triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene or diallyl phthalate; an ethylenically unsaturated mono- or dicarboxylic acid co-monomer or half ester thereof; and optionally an alkyl acrylate co-monomer.
In addition, U.S. Pat. No. 4,503,185 discloses aqueous vinyl ester/ethylene emulsion copolymers demonstrating enhanced dry pick strength and gloss in paper coating formulations. The copolymers comprise a vinyl ester/ethylene copolymer colloidally dispersed in an aqueous medium, the copolymer consisting essentially of (a) a vinyl ester of a C1-C12 alkanoic acid, (b) about 0.5 to 15 wt % ethylene, (c) 0.1 to 10 wt % of an ethylenically unsaturated C3-C10 mono- or dicarboxylic acid or half ester of such dicarboxylic acid with a C1-C18 alkanol, (d) 0.05 to 10 wt % of a dicyclopentadienyl acrylate, and (e) 0-50 wt % of an alkyl acrylate.
Notwithstanding the availability of these various types of paper coating binder dispersions, there is a persistent need for coated paper and coated paperbord producers to identify paper coating dispersions (i.e., paper coating binders) that provide, when used in paper coating compositions, increased binding strength when the compositions are applied to paper and paperboard products. Most often, binding strength is quantified by a paper test called the IGT pick strength test.
Evaluation of the picking effect exhibited by selected paper substrates coated with any given type of paper coating composition can be used to quantify the binding strength and coating performance of that composition. Picking evaluation is carried out by means of IGT pick testing according to standard methods of measurement by the Technical Association of the Pulp and Paper Industry (TAPPI) as well known in the art and incorporated herein by reference. A measure of dry binding strength is provided by the IGT dry pick test pursuant to TAPPI Useful Method UM 591, Surface Strength of Paper, the entirety of which is incorporated herein by reference. The IGT dry pick strength measures the speed, in cm/sec, required to lift the paper coating off of the surface of a paper substrate strip when printed using an ink roller and standard conditions as described in UM 591. Higher IGT dry pick numbers indicate better resistance of the coated substrate to picking and hence higher strength coating performance. Wet binding strength may be measured according to the IGT wet pick test, in which the wet binding strength is evaluated after the paper is wetted. An IGT damping unit includes a damping disc with doctor blade and a printing disk. The damping unit and doctor blade are used to apply a thin and consist amount of water to the paper substrate which is then printed with a standard IGT ink. After the ink is allowed to dry for 30 minutes, a densitometer is used to measure the density of the ink on the non-wetted printed surface and compared to that of the ink density on the pre-wetted and printed surface. The formula used for determining IGT wet pick is:
                                                                        Density                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                Nonwetted                ⁢                                                                  ⁢                Surface                            -                                                                          Density              ⁢                                                          ⁢              of              ⁢                                                          ⁢              Wetted              ⁢                                                          ⁢              Surface                                                  Density        ⁢                                  ⁢        of        ⁢                                  ⁢        Nonwetted        ⁢                                  ⁢        Surface              ×    100    =      IGT    ⁢                  ⁢    Wet    ⁢                  ⁢    Pick    ⁢                  ⁢    Value  Lower IGT wet pick numbers indicate better resistance of the coated substrate to picking and hence higher strength coating performance. A higher value indicated more wet pick.
A variety of emulsion polymerization components and techniques can influence binding strength, but, in general, vinyl ester-based binders (e.g., polyvinyl acetate, vinyl acetate-ethylene, vinyl acetate-acrylate, and the vinyl acetate-based binders discussed above) are known to provide lower binding strength than more commonly used coating binders like styrene butadiene (SB) and styrene acrylics. To compensate for the lower IGT pick resistance provided by paper coating compositions using such vinyl ester based binders, higher binder levels are typically required, which, of course, negatively impacts profitability of the coated paper and paperboard products made with these types of coating binders. In addition, high binder levels may result in adverse properties, e.g., reduced ink absorption.
A need therefore exists for vinyl ester-based copolymer dispersions that exhibit improved binding strength characteristics particularly when used in paper coating formulations.